On Tue, 10 Sep 2002 13:33:10 -0500 Jon Ogden <na9d@speakeasy.net> writes:
SNIP
>
> For a roof tower application, there is no reason why you shouldn't
> be able to find a wire length that is not resonant at HF and guy
> your tower with that length.
>
> 73, Jon NA9D
>
HF covers 3 to 30 MHz with corresponding wavelengths of
328 ft down to 32.8 ft for 1 wavelength (WL) or 164 ft to 16.4 ft
for a 'resonant' HALF WL. This means that to avoid resonance
in the HF range, any conductor must be *sufficiently* LESS THAN
16 ft long. While modeling a 2L Yagi, I determined that a parasitic
element must be Less Than 0.3 WL long to become nearly
'invisible' to the Driven Element. This corresponds to 10.5 ft
for a 10M antenna.
ANY conductor LONGER than 0.3 WL WILL couple to the DE
and act as either a Director if sufficiently shorter than 0.5 WL
or a Reflector if sufficiently longer than 0.5 WL.
Bottom Line: To create conductive guys that are INVISIBLE to HF
antennas, they must be broken up into segments less than ~10 ft
long between insulators. Note that at the tower, the first insulator
must then be placed less than 5 ft from the tower because of continuity
through the tower.
Sorry to keep repeating this message, but the idea does not seem
to be getting through. The 'out of band' resonances suggested in the
ARRL Antenna Books for the last 30+ years REDUCE coupling but
do NOT eliminate it. Such lengths are NOT non-resonant, they are
resonant at frequencies between the 'old bands', right where the
New (WARC) Bands fall. OLD Beliefs die hard.
Tom N4KG
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